In the installation of large and sometimes complex tooling where great precision and dimensional control is required, it is customary practice to position such tooling using theodolite and photogrammetric optical targets to achieve dimensional control of engineering or component features. The procedure is also used for periodic inspection.
So-called cup mounts, sometimes referred to as candlesticks, are used to provide the control points from which orientation is derived or alignment checked. They also facilitate a line of sight that establishes a method of coordinating features located on the tooling fixture. Such cup mounts are monuments or permanent features in the baseline for construction or validation. The cup mounts are designed to accept standard optical spheres which in turn hold see-through targets. These optical spheres are really hollow spheres with opposite and equal segments removed so that the target may be inserted therein.
The current line of sight spherical mounts were designed originally to be used with alignment scopes and conventional type optical transits and levels. These instruments are single axis devices relying on the mount being in a parallel or 90.degree. axis to the optical instrument. The introduction of electronic theodolites and photogrammetric measuring processes require target and instrument positions that necessitate a mutual viewing from various angles and attitudes by multiple instruments. In this regard, the conventional mount was severely limited because the outside diameter of the sphere limited the disk target from being viewed simultaneously by multiple instruments. This limitation of viewing angle presents a problem for processes and instrumentation requiring specific included or apex angles as related to electronic theodolite systems. The standard 2.250 inch diameter bore in the spherical mount that houses the disk-type target acts as an obstruction. This obstructed viewing is created when the disk target is inserted the required 0.750" necessary for the target center of the disk to become concentric with the spherical center of the mount. To facilitate setting this concentricity, a special gauge is used which must be set to the machined face of the current spherical-type mount. Special care must be taken when performing this operation.